It is desirable to print images on various substrates, including in packaging applications. In such applications, the desired image may be formed via ink jet printing. In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. An aqueous inkjet printer employs water-based or solvent-based inks in which pigments or other colorants are suspended or in solution. Once the aqueous ink is ejected onto an image receiving surface by a printhead, the water or solvent is evaporated to stabilize the ink image on the image receiving surface. When aqueous ink is ejected directly onto media, the aqueous ink tends to soak into the media when it is porous, such as paper, and change the physical properties of the media. Because the spread of the ink droplets striking the media is a function of the media surface properties and porosity, print quality will be inconsistent. To address this issue, either properties of the ink must be modified or properties of the media on which the inks are printed must be modified. For example, adding surfactants to the ink reduces the surface tension of the ink, but such solutions present additional problems including uncontrolled spreading of the ink. This can result in the edges of single pixel lines to be undesirably wavy. Moreover, aqueous printheads have certain minimum surface tension requirements (i.e., greater than 20 mN/m) that must be met for good jetting performance. Alternatively, protective base-layers can be formed on the media by ejecting curable inks onto the media surface and then curing the curable inks, for example, via ultraviolet radiation for UV curable inks. The subsequently printed ink droplets may then be printed on the base-layers in a manner that avoids the changes in image quality resulting from the media properties, such as those that occur in response to media contact with the water or solvents in aqueous ink and affect ink drop spread.
As such, the base formed by the cured inks are typically referred to as the image “under-layer” which may comprise one or more layers of white, transparent or colorful inks. Use of these under-layers can be particularly important when printing onto plastics, especially those that are clear or translucent. While the base is necessary in order to achieve desired final image color and overall product appearance to avoid ink-media interactions, one issue that arises with printing onto a UV Ink base or under-layer is the inability of the subsequently color printed image to adequately wet it. Such under layers may also be susceptible to other issues. For example, if the under layer is under-cured, it may offset to any other surface that it comes into contact with (e.g., rollers used in the printer to transport the media). It is also possible that in an under cured state the subsequently printed color ink drops may be enveloped, encapsulated or otherwise distorted by the soft under-cured under-layer(s). Likewise, over-curing of the base ink under-layer may also lead to unacceptable results, such as insufficient wetting of the subsequently printed image color layers.
While optimization of curing parameters could be relied on to address these problems, other problems arise. For example, depending on the image being formed, the curing parameters for optimal curing of the base layer may be different from image to image, may vary depending on the different media being printed on or image forming inks being deposited thereon, or may simply not be attainable and risks offsetting as described above when the underlayer(s) contact a surface of the printer. Therefore, optimization of curing parameters to cure the UV inks that form the base under-layers is not feasible.
Surfaces that provide for improved image formation without adding to the issues of curing the base layer(s) are desirable. Therefore, there is a need for an approach for controlling UV-curable ink surface properties and subsequently printed ink-spread of a printed image during a printing process.